This is a quote about the situation just before the Civil War, when the transportation revolution had created a situation in which railroads and canals and such allowed goods to be traded relatively easily between areas of the country that were not previously connected.
The political implications of this have to do with the regional conflicts that led to the Civil War. The South thought that the Mississippi River connected them economically to the states of what is now the Midwest because those states had to send goods down the river to the South. They expected that those states would secede with them. However, the "national market" the quote mentions made this thinking incorrect. The Midwest was now linked to the East instead of the South. The railroads and canals allowed this linkage to happen. Because of it, the Midwest allied itself with the North when the crisis erupted between the North and South.